A polycarbonate resin has been applied to various optical materials because it has high transparency, excellent heat resistance and good dynamic properties. Particularly, it has been known that when optical materials such as optical films, optical discs, optical prisms and pickup lenses have large birefringence, various problems such as fuzzy phenomenon in an image formation point of a light transmitted the inside of the material and reading error of information occur. Thus, resins with small birefringence have been developed.
For example, Japanese Patent Kokai (Laid-open) Nos. 6-25398 and 7-109342 disclose an investigation of a whole aromatic polycarbonate resin copolymer used bisphenols having a fluorene structures with large polarizability in the direction of side chain.
Japanese Patent Kokai (Laid-open) Nos. 10-101787 and 10-101786 disclose a homopolycarbonate resin of ether diols having a fluorene structure with large polarizability in the direction of side chain and a phenol skeleton in the direction of normal chain aimed to attain lower photoelastic constant and a copolymer thereof with bisphenols.
Further, Japanese Patent Kokai (Laid-open) No. 2000-169573 suggests, as described in Examples 4, a copolymer of bisphenols having a fluorene structure with large polarizability in the direction of side chain and tricyclodecane [5.2.1.02,6] dimethanol.
As described above, various materials with low birefringence have been developed. The development aims to attain lower photoelastic constant. On the other hand, occurrence of birefringence is not only due to photoelasticity, but also due to molecular orientation during molding or due to stretching. A molded article with such hysterisis has birefringence prior to applying a stress. Birefringence due to deformation hysterisis is generally large than that due to photoelasticity. Since materials are preliminarily subjected to molding or stretching, development for a resin to exhibit low birefringence has been required, even if it is subjected to any operation.